1. Field of the Invention
This invention relates to a method of fabricating ceramic multilayer substrates by laminating green sheets.
2.Description of the Prior Art
Reduction in planar or X-Y shrinkage of ceramic substrates during firing has recently been desired for improvement in dimensional accuracy of the substrates. PCT WO91/10630 discloses a method of fabricating ceramic substrates in which a green sheet to be formed into an insulating layer of a ceramic substrate is provided with two release green sheets stacked on upper and lower surfaces thereof respectively. Each release green sheet is unsintered at a firing temperature of the green sheet. The green sheet is fired with the stacked assemblage under pressure. Thereafter, the unsintered release green sheets adherent to both surfaces of the fired assemblage are removed, so that the ceramic substrate is obtained.
The ceramic substrate is often fabricated into a multilayer structure by laminating a plurality of green sheets. The ceramic multilayer substrate has conductor patterns formed on inside layers thereof. The surfaces of the substrate are deformed such that portions thereof corresponding to the conductor pattern 14 are outwardly convexed when, as shown in FIG. 4, a pressure is applied to a laminate of green sheets 11 with two release green sheets 12 placed on opposite surfaces of the laminate so that the laminate and the release green sheets 12 are bonded together, or when, as shown in FIG. 5, a bonded laminate of green sheets 11 is fired together with the release green sheets 12 placed on the opposite surfaces of the laminate while a pressure is being applied to the laminate by a pressing machine 13. As a result, surface flatness or planarity of the substrate is disadvantageously reduced. The reduction in the substrate surface flatness lowers the reliability of conductor patterns which are printed on the surfaces of the substrate and fired after firing of the substrate, or causes reduction in the reliability in connection of an IC chip to be mounted on the surface of the substrate.